A method was developed to determine the control response of a single-rotor helicopter equipped with unbalanced, flexible blades. The parameters of a typical 5000-lb helicopter were assumed, and the results were compared with those utilizing the assumption of rigid blades. The principle effect of the overbalanced blades (cg forward of the elastic axis) was an increase in the damping in pitch of the helicopter. Underbalanced blades increased fuselage oscillations and decreased the periods as a result of a decrease in pitch damping. A time lag was introduced by adding damping to the twisting motions of the blades, and blades with a response component in phase with attitude as well as pitching rate were considered possible. The neglect of blade flexibility and unbalance appeared to reduce materially the effectiveness of an automatic control system. Aerodynamically unbalanced blade effects were similar to those for mass unbalance. A detailed mathematical analysis is included as well as a simplified analysis suitable foe engineering requirements.